Influence of Current Density upon Hydrogenation on the Shape Memory Effect of Binary TiNi Alloy Single Crystals

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m(2) for 3 h at room temperature on the temperature dependence of the yield stress sigma(0.1)(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m(2), respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the M-s temperature of the onset of the forward martensitic transformation (MT) upon cooling and the M-d temperature (M-d is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress sigma(0.1) of the B2 phase at the M-d temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME epsilon(SME) = 10 +/- 0.2% at sigma(ex) = 200 MPa and epsilon SME = 10.5 +/- 0.2% at sigma(ex) = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m(2), respectively, which exceeded the theoretical value of lattice deformation epsilon(0) = 8.95% for the B2-B19' MT in [011] orientation under tension. At j = 1500 A/m(2), the physical reason for the excess of the SME of the theoretical epsilon(0) value was due to the increase in the plasticity of B19' martensite upon hydrogenation. At j = 3500 A/m(2), epsilon(SME) = 8.0 +/- 0.2%, and it was less than epsilon(0) = 8.95% for B2-B19' MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m(2) was associated with the interaction of two types of B19' -martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m(2) increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100-150 MPa. After long-term holding, epsilon(SME) = 9.5 +/- 0.2% at 150 MPa, which exceeds the theoretical value epsilon(0) = 8.95% for B2-B19' MT in [011] orientation under tension.